1. Field of the Invention
The present invention relates to electrical networks, and, more particularly, to an electrical coupler network.
2. Description of the Related Art
Electrical signals can be divided and/or combined for a variety of purposes. For example, the signal power output requirements for a radio frequency (RF) system may exceed the capability of readily available RF amplifiers. To produce the required power output, the RF signal is divided and inputted to multiple amplifiers. The individual amplifier outputs are then combined to provide a power output which none of the amplifiers can produce individually.
A hybrid coupler is used for generating or combining near equal amplitude signals, having a predetermined relative phase, in the communications and related fields such as radar, navigation, etc. Hybrid couplers are used in many RF circuits and systems. The hybrid coupler is also known in the literature as the 3 dB hybrid. A hybrid coupler differs from a directional coupler, for example, in that the hybrid coupler can split an input signal into two nearly equal amplitude signals whereas the directional coupler splits the input signal into two unequal amplitude outputs.
The hybrid coupler is a four port device that can function both as a power splitter and as a power combiner. Two ports function as inputs and the other two ports function as outputs. When used as a power splitter, one of the input ports (commonly named the isolated port) is terminated in a resistance matched to the system's characteristic impedance (Z0, typically 50 ohms for RF signals). FIG. 1A illustrates a schematic symbol for a hybrid coupler and FIG. 1B illustrates a corresponding phase truth table for the hybrid coupler. Referring to FIG. 1B, for a signal input at port A, for example, port B is the isolated port terminated with characteristic impedance Z0, port C outputs a signal with an approximately 0° relative phase with respect to the signal input at port A, and port D outputs a signal with an approximately 90° relative phase with respect to the signal input at port A. FIG. 1B therefore illustrates what is sometimes referred to as a 90° hybrid.
Unfortunately, real hybrid couplers have non-ideal amplitude and phase responses that vary with operating frequency. Amplitude imbalance is defined as the difference between the hybrid coupler's two output signal amplitudes when operated as a splitter. Phase error is defined as the relative phase deviation from 90° between the hybrid coupler's two output signals when operated as a splitter. FIG. 2 shows a model of a real hybrid coupler functioning as a power splitter. In FIG. 2:                VI=input signal amplitude (volts)        VO=output signal amplitude (volts)        ω=signal angular frequency (radians per second)        t=time (seconds)        ε=amplitude imbalance        θ=phase error (radians)Although approximately one-half of the power is delivered to each output, the two outputs do not have identical amplitudes as indicated by the (1+ε) term in FIG. 2. The phase error is indicated by the θ term of FIG. 2. Further, the amplitude response and the phase error of the coupler vary according to the frequency of the input.        
Amplitude imbalance and phase error typically limit the useable frequency bandwidth of hybrid couplers to about one octave (2:1 frequency bandwidth, ratio of highest to lowest frequencies). Both amplitude imbalance and phase error are a function of frequency. A typical hybrid coupler can have amplitude imbalance as great as ±1 dB and phase error as great as a few degrees over a 2:1 frequency bandwidth.
For example, a typical hybrid coupler can have a 90° output with an amplitude output of −2.6 dB located at the center frequency of the input, and the 0° output can have a amplitude output of −3.4 dB at the center frequency, both in contrast to an ideal output of −3.0 dB. When one coupler drives two other couplers to create a four-way power divider, the imbalance at the four outputs of the two driven couplers is typically ±0.8 dB. One output is typically at −5.2 dB and the other output is at −6.8 dB, both in contrast to an ideal −6.0 dB. If these divided signals were sent to four amplifiers for amplification, one of the amplifiers would be presented a signal at approximately −5.2 dB, that is approximately 30% of the input signal power, rather than the desired 25%. As identical amplifiers are often used, each amplifier is required to be sized to handle 30% of the input signal value. This requirement limits amplifier selection, requires greater amplifier capacity, and reduces reliability due to one amplifier is amplifying an excess signal that should, ideally, be shared among four amplifiers.
Adding a fourth coupler can solve some of the amplitude imbalance problems; unfortunately, phase errors are associated with this type of solution. These phase errors contribute to amplitude errors which are significant enough to negate the amplitude enhancement when the four amplified signals are recombined.
The bandwidth limitation described above for the hybrid coupler can be the principal bandwidth limiter for a circuit in which the hybrid coupler is used. Additionally, such a network is susceptible to electromagnetic interference (EMI) and common mode noise. Electromagnetic noise coupled into the input can produce a common mode noise signal on the outputs thereby reducing the signal to noise ratio (S/N) of the circuit.
U.S. Pat. No. 5,313,174 (Edwards) discloses a 2:1 bandwidth RF splitter/combiner, that when used as a splitter, produces four signals. The Edwards '174 splitter/combiner improves some of the amplitude imbalance and phase errors present in the constituent hybrid couplers. Unfortunately, the Edwards splitter/combiner does not produce four signals sequentially offset by 90° and therefore is not a balanced circuit. The Edwards '174 splitter/combiner produces four signals with 180°, −180°, 90°, and −90°. However, this represents only three distinct electrical phases (180°, 90°, and −90°), since a signal with −180° electrical phase cannot be distinguished from a signal with 180° electrical phase. Further, the Edwards '174 splitter/combiner has only a single input port when operated as a splitter, it therefore cannot reject common-mode signals and is more susceptible to EMI. Yet further, the Edwards '174 splitter/combiner has only a 2:1 frequency bandwidth which is no better than the constituent hybrid couplers that make up the circuit.
U.S. Pat. No. 6,445,346 (Fathy et al.) discloses a polarizer feed network for a dual circular polarized antenna array. However, the Fathy et al. '346 network does not cancel the amplitude imbalance and phase error present in its two branch line couplers. Further, the Fathy et al. '346 does not have an extended frequency bandwidth beyond that of its constituent branch line couplers.
What is needed in the art is a network that cancels out the amplitude and phase errors inherent with hybrid couplers and thereby produces or combines equal-amplitude quadrature-phase signals with greater accuracy. Further, what is needed is to increase the frequency bandwidth over which equal-amplitude quadrature-phase signals can be generated or combined. Yet further, what is needed is a hybrid coupler network that is electrically balanced with respect to ground, and which can reciprocally operate.